Composites are defined broadly as the combination of two or more dissimilar materials to produce a new material that has synergistic properties that were not present in the individual constituents alone. In practical terms, the word composite is generally associated with reinforced plastic material such as fiberglass structures. In the case of fiberglass, beneficial synergistic properties including corrosion resistance, low weight, high strength, and low cost are attainable in a highly variable array of product geometries.
Fabrication of a composite article such as a fiberglass boat hull, bridge deck, or wind blade requires the combination of a solidifiable resin system with a “preform” that could include various reinforcements such as glass fibers, veils, flow media, and cores. There are many processes available for the purpose of impregnating a preform with liquid resin in order to make a composite. One such process is resin transfer molding, which involves a preform being constrained between molding surfaces whereupon resin is introduced into the open spaces remaining. Resin infusion methods advantageously limit exposure to Volatile Organic Compounds (VOC) and Hazardous Airborne Pollutants (HAP), which are recognized by the Environmental Protection Agency (EPA) as potential health hazards for which alternative control technologies should be sought. Further, resin infusion methods allow for better control over part dimensions and fiber volume fraction.
Vacuum Assisted Resin Transfer Molding (VARTM) is a specific approach to resin transfer molding that achieves preform compaction by removing air located between a single sided rigid tool and a flexible vacuum bag that encapsulates a preform placed on the tool. Tooling costs are significantly reduced because there is only one tool surface, and atmospheric pressure replaces the hydraulic press. VARTM provides an affordable closed mold solution for many parts that were previously not considered infusable.
Vacuum bags intended for both vacuum infusion and debulking rely on an ability to remove air and other fluids that reside inside the cavity formed between the mold and the bag. This is typically accomplished by connecting a vacuum pump to one or more vacuum ports located either in the base mold or through the vacuum bag. When the bag is in an initial uncompressed state, it is relatively easy to remove fluids through the discrete vacuum ports. However, as the bag is evacuated, the force of atmospheric pressure above tends to flatten the bag against the base mold and can thus make it progressively more difficult to remove any remaining fluids from between the molding surfaces. Current solutions to this issue generally involve placing pieces of a porous “breather” material in strategic locations throughout the molding surface so as to provide active vacuum channels even when the bag is collapsed on the mold surface. This process requires manual labor to set up, however, and when waste fluids come in contact with the breather material, the material often needs to be replaced.
A further issue regarding the use of vacuum bags is the need to quickly achieve an effective perimeter seal. Common sealing systems include the use of tapes, adhesives, or reusable fasteners. Each of these solutions has problems, though, including the need for additional labor and the creation of material waste. In addition, vacuum bags tend to change shape during a typical life cycle, and therefore the location of fixed seals may move in time making it difficult to maintain bag tension, engage the seals, or both.
In light of these factors, it is apparent that despite a growing interest in the use of reusable vacuum bags, a number of hurdles exist to reusable vacuum bags being fully embraced by the composite industry. Specifically, there exists a need for a vacuum bag that can provide a secure seal to a base mold while still allowing as much vacuum for as long as possible so as to remove air from multiple locations on the dry preform and uniformly infuse the preform without wastefully draining large volumes of resin from the preform.